Remove broken attempt at parsing date-only

Remove broken attempt at parsing date-only

g_time_val_from_iso8601 was attempting to parse strings
having only a date, but failed to actually set the timeval
despite returning TRUE. Since the docs state that the function
only parses strings containing a date and a time, just return
FALSE in this case.

Also remove an incomplete testcase for this behaviour that was
just checking the boolean return value, but not timeval.